Porous ceramic

ABSTRACT

Porous ceramics are described, which are produced by 
     a) mixing an aqueous solution of a suitable ionotropically orientable polyanion, either with 
     oxides, hydroxides or hydrated oxides, which are present in the form of a sol, of the metals Al, Zr, Ti and Nb, 
     or with finely crystalline oxides, hydroxides or hydrated oxides, which are present in suspension, of these metals, 
     or with finely crystalline tricalcium phosphate or apatite which are present in suspension, 
     b) bringing the mixed sol obtained as in a) or the suspension obtained as in a) into contact with a solution of a salt of a di- or trivalent metal cation in order to produce an ionotropic gel body, 
     c) compacting the gel body by introducing it into electrolyte solutions which further enhance the syneresis of the polyelectrolyte which was originally formed, 
     d) washing the gel body with water and subsequently impregnating it with a readily volatile, water-miscible solvent, 
     e) freeing the anhydrous gel body or gel bodies obtained as in d) from the readily volatile, water-miscible solvent, 
     f) burning out the organic constituents from the dry gel body or the dry gel bodies obtained as in e), 
     g) sintering the product obtained as in f). 
     A capillary frit is also described. Moreover, the invention describes the use of these materials as a catalyst or as a catalyst support, as a ceramic component for composite materials, as a reversible flow filter and as a slab-like sorbent for chromatography columns, as well as the use of a composite material, which is produced from a granular material of the ceramics which are described, as a dental material, particularly as a dental cement.

This invention relates to a porous ceramic and to a process forproducing it.

Highly porous ceramics are increasingly being used as filter systems, asimplants in medical technology and as supports for catalysts. There arenumerous processes for producing them, only two of which will be citedhere:

The production from aerogels and the burning-out of previously admixedorganic additives.

The areas of use of ceramics such as these depend on their chemical andthermal stability, on their permeability, on their specific surface andon the length of the diffusion paths to their active surface.

The materials are selected based on requirements imposed by their areaof use and stability; their pore size, pore size distribution and poreshape are selected based on the requirements of permeability, diffusionpath and surface area. The importance of these properties to the use ofthese ceramic substances is clear: good stability enables them to beused even with aggressive media and at high temperatures, whilst goodpermeability results in a low pressure drop during operation and thusfacilitates low energy consumption; a high specific surface results in ahigh density of adsorption centres and/or reaction centres; shortdiffusion paths enable active centres to be reached by flows of materialin reasonable timescales.

Unfortunately, only partial success has been achieved hitherto inoptimising these four important properties simultaneously in order toobtain high stability, a low resistance to flow, a high specific surfaceand short diffusion paths. Good permeability cannot be achieved withrandomly oriented powdered materials or random arrangements of materialswhich are subsequently sintered, because these are macroscopicallyisotropic and also exhibit a high resistance to flow (one known exampleof a sintered, randomly oriented powdered material is a glass frit whichis fused into chemical apparatuses). Instead, macroscopicallyanisotropic arrangements of particles are required, and what is requiredin practice is therefore systems of tubes or capillaries which are openat both ends and which have aperture diameters of the same magnitude.However, it has hitherto only been possible to produce systems such asthese with tube and/or capillary spacings which are large in absoluteterms. The regions between the capillaries/tubes can only be reached vialong diffusion paths. For this reason, a high specific internal surfaceof the wall material is incapable of having the desired effect, since itis only the edge regions thereof around the tubes/capillaries which canbe reached by flows of substance within a reasonably short timescale.

Currently, ceramics through which capillaries or tubes pass can only beproduced by extrusion methods. The smallest diameter which can therebybe achieved is 200 μm. The spacings between the capillaries/tubes areabout 600 μm. commercially available. Even the patent and scientificliterature contains no references to structures such as these.

It is only the document EP-A-0479553 that describes porous ceramics witha high porosity and a narrow pore size distribution which are obtainedby preparing a dilute slurry of a ceramic starting material in asolution of a high molecular weight organic compound such as ammoniumalginate, which can be converted into a gel by reaction with an acid orwith tri- or polyvalent cations or by heating or cooling. The slurry isbrought into contact with a liquid or with a gel in which the acid orthe tri- or polyvalent cations are present, or is heated or cooled, inorder to obtain a ceramic gel substance which is subsequently calcined.These porous ceramics exhibit improved resistance and mechanicalstrength and are thus suitable as high-temperature catalyst supports.

The underlying object of the present invention is therefore to provide aceramic body with a high stability, a low resistance to flow, a highspecific surface and short diffusion paths, as well as a process for theproduction thereof.

The present invention relates to a porous ceramic, which is produced by:

a) mixing an aqueous solution of a suitable ionotropically orientablepolyanion, either with

oxides, hydroxides or hydrated oxides, which are present in the form ofa sol, of the metals Al, Zr, Ti and Nb,

or with finely crystalline oxides, hydroxides or hydrated oxides, whichare present in suspension, of these metals,

or with finely crystalline tricalcium phosphate or apatite which arepresent in suspension,

b) bringing the mixed sol obtained as in a) or the suspension obtainedas in a) into contact with a solution of a salt of a di- or trivalentmetal cation in order to produce an ionotropic gel body, solution of asalt of a di- or trivalent metal cation in order to produce anionotropic gel body,

c) compacting the gel body by introducing it into electrolyte solutionswhich further enhance the syneresis of the polyelectrolyte which wasoriginally formed,

d) washing the gel body with water and subsequently impregnating it witha readily volatile, water-miscible solvent,

e) freeing the anhydrous gel body obtained as in

d) from the readily volatile water-miscible solvent,

f) burning out the organic constituents from the dry gel body obtainedas in e),

g) sintering the product obtained as in f).

Ionotropic gels are formed when a dilute aqueous solution of a suitableanionic polymer, for example a solution of a sodium alginate or of asodium pectinate, or of sodium cellulose xanthogenate, sodium xanthateor sodium hyaluronate, is brought into contact with a solution of adivalent cation such as Cu²⁺ or Ca²⁺ or with a solution of a trivalentcation such as Al³⁺ or La³⁺. This is effected, for example, by addingthe solution of the polyanion drop-wise to the solution of the metalcation or by adding the solution of the metal cation drop-wise to thesolution of the polyanion, or by coating one solution with the other inthe absence of convection. The proportion by weight of the polyanion inthe sol can range between 0.25 and 5.0 percent by weight. Proportions byweight from 0.5 to 2.0 percent by weight are particularly suitable. Theconcentrations of the metal salt solutions are greater than 10⁻³ M andare less than the respective saturation concentration of the salt inwater. Concentrations between 10⁻¹ and 2 M are most suitable. After theformation of a membrane-like precipitate at the phase boundary betweenthe two liquids, which is termed the primary membrane, orienteddiffusion occurs of the low molecular weight electrolyte into thesolution of the polymer. However, the precipitation which continues tooccur at this location does not result in amorphous precipitate, but ina gel which is structured in three dimensions.

Regularly arranged capillaries are then formed, which are of practicallyidentical size and which are circular in cross-section, the walls ofwhich capillaries consist of the precipitated product and the lumina ofwhich absorb the water evolved during precipitation. The stability ofthe gel is therefore based on the fact that the di- or trivalent cationscrosslink the molecules of the polymer with each other and thus impart acertain mechanical strength to the capillary walls. The capillaries areall parallel to each other in the direction of diffusion of theelectrolyte and can reach a length of a few centimeters. The arrangementof the capillaries is almost perfectly hexagonal and their radii slowlyincrease in the direction of diffusion of the metal cation, with agradient of about 5%. The diameter of the capillaries of the gel can beadjusted within wide limits via the viscosity of the polyanion and thetype of polyvalent cation. The lower limit which could be achievedhitherto was about 5μm, and the upper limit was about 300 μm. If thegels are produced by coating the two solutions, their strength in mostcases is sufficient to enable them to be cut—starting at the top—intoslices, the smallest thickness of which is about ½ mm and maximumthickness of which can be about 2 cm. Due to the conicity of thecapillaries, which are perpendicular to the slices, capillaries witharbitrarily predeterminable diameters can be obtained in the gel,depending on the depth of cut.

The alginate which is cited as an example here has been investigatedparticularly thoroughly with regard to its capacity for ionotropicorientation. Moreover, the capillary gels of this alginate areparticularly regular. Under comparable experimental conditions, however,other substances, which were not investigated in such detail, also formpatterns of capillaries which only exhibit qualitative differences fromthose of the alginate.

Amongst others, these latter substances include organic polyions such aspectinates, cellulose xanthogenates, xanthates, hyaluronates,chondroitin sulphates, salts of carboxymethyl cellulose, of carboxylcellulose and of chitosan; they also include complexes of polyanions(symplexes) and polycations, and finally they also include inorganicsubstances such as vanadium pentoxide and mixed organic/inorganicsubstances such as mercury sulphosalicylate. These substance aretherefore all polyelectrolytes.

According to the invention, it is possible to use ionotropic capillarygels as template structures for the production of crack-free, porousceramics, the geometric structure of which is completely identical tothe corresponding structure of the gels. It is only the dimensions ofthe porous ceramic structures as a whole which are somewhat smaller thanthose of the gel, as a result of drying and sintering. At first glance,the external appearance of ceramics such as these is the same as that ofporous sintered bodies made of glass. As distinct from the latter,however, the ceramic is not isotropic, but is highly anisotropic as aresult of the capillaries. On a second glance, this can be recognised bythe iridescent sheen of the ceramic surface, which is reminiscent of theappearance of facets.

The diameters of the capillaries of the ceramic have values between 1and 200 μm, and the spacings between the capillaries are about the samesize as the capillary diameter. The free surface of the ceramic, whichis formed by the capillary apertures, amounts to about 50% of its totalsurface area, and there are about 50,000 capillaries per cm² of surface.The porosity or proportion of voids of the ceramic ranges between 75%and 85% of the total volume thereof.

The porosity of the ceramic can be analysed in greater detail by meansof mercury porosimetry. It is found that a ceramic produced fromalginate/boehmite mixed sols, for example, has a trimodal pore sizedistribution, whereas a ceramic obtained from suspended alumina has abimodal distribution. In both these cases, the capillaries constitutethe population of the largest pores and the interstices between thealumina particles of the pore walls constitute the population of thesecond largest pores. Moreover, when boehmite is used the spaces betweenthe individual crystallites of boehmite constitute a third population,which is manifested as an additional internal porosity of the aluminaparticles in the capillary walls. The first population (capillaries) hasa pore size maximum at about 10 μm, the second at about 130 nm, and thethird at 58 nm.

Production of the ceramic necessitates that either a sol of a metalhydroxide or of a hydrated metal oxide is produced in the alginate solor that a slurry of a finely crystalline metal oxide, metal hydroxide orhydrated metal oxide is admixed with the alginate sol.

The term “finely crystalline” denotes average particle sizes from 10 nmto 1 mm, preferably from 100 nm to 500 nm.

The formation of the ionotropic capillary gels from mixed sols proceedsexactly as does the formation thereof from unmodified sols, except thatthe inorganic component accumulates in the capillary walls. In thelatter gel bodies, the density of the solid particles in the capillarywalls is still so low that a ceramic produced therefrom does not yetpossess the desired mechanical strength even after being subjected toall the other process steps. It is therefore necessary to subject thegel body to a shrinkage process, in order further to increase theparticle density. For this purpose, the syneresis which occurred withthe original formation of the gel has to be further enhanced, andadditional proportions of the water which is bound in the gel have to bedischarged in the capillary lumina. In order to achieve this, there arevarious practical options which are all theoretically based on the sameprinciple. The osmotic pressure in the regions of the capillary wallshas to be reduced, and the osmotic pressure in the lumina has to beincreased. Increasing the pressure in the lumina does not necessitatefurther discussion; reducing the pressure in the regions of thecapillary walls is effected by ion exchange. The polyvalent metalcations which were originally required for gel formation are replaced bymore strongly bound ions, which in the best case are even more weaklyhydrated. Amongst other type of ions, the latter can be protons, withthe gel body being incubated in baths of hydrochloric acid of increasingconcentrations, for example, in order to effect further compaction. Inorder to effect the exchange of metal ions, it is particularly effectiveif a solution of a poly- or oligo-electrolyte, the solid-state ions ofwhich bear the opposite charge to the solid-state ions of the polyionwhich was originally formed, is introduced into the capillary lumina. Inparticular, oppositely charged, high molecular weight electrolytes aremutually precipitated to form precipitates of high density, which areinsoluble in most solvents. These are termed “symplexes”. If the polyionwhich was originally gel-forming is a polyanion, an alginate, forexample, the ions which are suitable for symplex formation include allpolycations, e.g. polyvinylamine, polyallylamine, polyvinylpyridine orpolyethylene-imine cations, and also include polycations of biologicalorigin such as chitosan; examples of suitable oligocations includepentaethylenehexamine or, again, biological substances such asprotamines and histones. Charged oligomers and polymers which aresuitable for symplex formation also include micelles of ionicsurface-active agents.

For symplex formation also, it is advantageous to increase theconcentrations of the precipitating polyions by adding the latter inportions.

During ion exchange, the gel bodies shrink in every dimension to abouthalf their original linear dimensions, and ion exchange has to becarried out particularly carefully since the gel substances exhibit apronounced tendency towards crack formation during shrinkage.

After ion exchange, the gel substances are subjected to a series ofwashing and dewatering steps in which water is displaced by a readilyvolatile organic, water-miscible solvent, e.g. acetone. This solvent isalso removed after dewatering. Examples of processes which are suitablefor this purpose include supercritical drying in carbon dioxide,pressing out the liquid with the aid of fine-pored earthenware slabs, orsimply allowing the batch to stand in air under ambient conditions.After the solvent has been removed, a typical “green body” is formed.This is heated according to an appropriate temperature-time programme,whereupon the organic template structure is carbonized or burned.Finally, the finished ceramic is formed by sintering. The organictemplate substances are routinely burned out at 600° C. for two hours.The completion of burn-out in each case is indicated by the colour ofthe sample changing from black to white. Temperatures of 800° C. to1400° C. are required for sintering the samples. The duration of firingranges from two to ten hours. The temperature and duration of the firingoperation are ultimately determined based on the magnitude of thespecific surface of the ceramic and the mechanical performance thereof,and are re-determined from case to case depending on the requirementsimposed on the final product.

The ceramic according to the invention can be produced in variousgeometric shapes, depending on the sphere of use thereof. The ceramicscan be employed for particularly diverse uses if they are in the form offrits of defined geometry which can readily be incorporated inpredetermined apparatuses or parts of apparatuses. Frits such as theseare plate-shaped and can be produced in both round and polygonal form.Their thicknesses range from 2 to 4 mm. The edge lengths of polygonalplates are about 100 mm long, as are the diameters of round discs.

Another form of the ceramic consists of a granular material comprisingpredominantly spherical grains a few millimeters in diameter. This isobtained by the drop-wise addition of one of the two solutions involvedto the other, followed by the appropriate further processing of thesmall spheres of gel which are thereby formed.

It is sometimes sufficient simply to comminute the ceramic mechanicallyand then to obtain various sieve fractions therefrom. It is importanthere that the particle dimensions remain significantly larger than thepore diameter, because each particle should contain a plurality ofcapillaries.

The capillary ceramic can be employed for diverse uses, particularly forthe use of frits or granulated material as a catalyst or catalystsupport. This applies in particular to ceramics comprising the oxides ofZr, Ti, Nb or of Al.

In the form of a frit, the ceramic can also be employed as a permeablesorbent for chromatographic purposes. A ceramic made of alumina issuitable for this purpose.

If the ceramic consists of apatite or tricalcium phosphate, it can beused in the surgical field as a porous template structure for thereplacement/new formation of bone.

If the ceramic is comminuted in a suitable mill so that the resultingparticles are no smaller than about 100 μm and thus still contain anumber of intact capillaries, an inorganic material is available whichcan be processed together with organic polymers, for example acrylicresins, to form composite materials with interesting new properties, foruse as dental fillings for example. If the porous capillary ceramic(optionally after covering the surface thereof with a coupling agent) isimpregnated with monomers or oligomers of a polymerisable substance, theshrinkage which unavoidably occurs in the interior of the capillariesduring polymerisation is particularly low. Consequently, dental fillingsmade of a composite material which contains a high proportion ofcapillary ceramic only exhibit a slight tendency to form cracks due toshrinkage between the dentine and the filling on curing.

The invention is explained in more detail by the examples given below.

EXAMPLE 1

A ceramic capillary frit produced from a mixed sol consisting ofboehmite and sodium alginate.

In a first step, a boehmite sol is produced. The proportion by weight ofboehmite in this sol is initially about 3%. In a second step, theboehmite sol is concentrated to a boehmite content of 16.5%. In a thirdstep, one proportion by weight of the concentrated boehmite sol is mixedwith four proportions by weight of a 6×10⁻² M solution of sodiumsulphate. After homogenisation, one proportion by weight of thissulphate-containing suspension is introduced into one proportion byweight of a sodium alginate sol, which contains 2% by weight of sodiumalginate, and is again homogenised therein. At this point, a fairlystable boehmite/alginate mixed sol is present.

The subsequent steps comprise: coating the mixed sol with 1 M Cu(NO₃)₂in the absence of convection, allowing the gel to mature for 10 hours(i.e. forming the capillary structure), cutting gel slices, effectingion exchange (Cu²⁺replaced by 2H⁺) by successively introducing the gelbody into baths of increasing hydrochloric acid concentration, washingout the remaining acid with water and replacing the water by acetone,removing the acetone by suction through porous earthenware slabs,burning out the organic constituents of the green body at 600° C. for 2hours, and sintering at 1400° C. for 2 hours to form the capillary frit.The ceramic body is cut to its final dimensions and is optionallylapped.

EXAMPLE 2

A ceramic capillary frit produced from a suspension of alumina in asodium alginate sol.

A fine crystalline alumina powder with an average particle size of 350nm is suspended in water. The proportion by weight of alumina in thissuspension is 8%.

One proportion by weight of this suspension is then mixed with oneproportion by weight of a sodium alginate sol (2% by weight). Afterhomogenisation, further processing was effected corresponding to Example1, starting with coating by a copper nitrate solution.

What is claimed is:
 1. A porous ceramic, obtainable by: a) mixing an aqueous solution of a ionotropically orientable polyanion, either with oxides, hydroxides or hydrated oxides, which are present in the form of a sol, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline oxides, hydroxides or hydrated oxides, which are present in suspension, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline tricalcium phosphate or apatite which are present in suspension, b) bringing the mixed sol obtained as in a) or the suspension obtained as in a) into contact with a solution of a salt of a di- or trivalent metal cation in order to produce an ionotropic gel substance, c) compacting the gel substance by introducing it into electrolyte solutions which further enhance the syneresis of the polyelectrolyte which has originally formed, d) washing the gel substance with water and subsequently impregnating it with a volatile, water-miscible solvent, e) freeing the anhydrous gel substance or gel substances obtained as in d) from the volatile, water-miscible solvent, f) burning out the organic constituents from the dry gel substance or the dry gel substances obtained as in e), g) sintering the product obtained as in f).
 2. The porous ceramic according to claim 1, characterized in that it is formed as a capillary frit and is obtainable by: a) mixing an aqueous solution of a ionotropically orientable polyanion, either with oxides, hydroxides or hydrated oxides, which are present in the form of a sol, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline oxides, hydroxides or hydrated oxides, which are present in suspension, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline tricalcium phosphate or apatite which are present in suspension, b) coating the mixed sol obtained as in a) or the suspension obtained as in a) with a solution of a salt of a di- or trivalent cation in order to produce an ionotropic gel substance and cutting said gel substance into slices, c) compacting the gel substance by introducing it into electrolyte solutions which further enhance the syneresis of the polyelectrolyte which has originally formed, d) washing the gel substance with water and subsequently impregnating it with a volatile, water-miscible solvent, e) freeing the anhydrous gel slices obtained as in d) from the volatile, water-miscible solvent, f) burning out the organic constituents from the dry gel slices obtained as in e), g) sintering the product obtained as in f).
 3. A process for producing a porous ceramic, characterized by the following steps: a) mixing an aqueous solution of a suitable ionotropically orientable polyanion, either with oxides, hydroxides or hydrated oxides, which are present in the form of a sol, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline oxides, hydroxides or hydrated oxides, which are present in suspension, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline tricalcium phosphate or apatite which are present in suspension, b) bringing the mixed sol obtained as in a) or the suspension obtained as in a) into contact with a solution of a salt of a di- or trivalent metal cation in order to produce an ionotropic gel substance, c) compacting the gel substance by introducing it into electrolyte solutions which further enhance the syneresis of the polyelectrolyte which has originally formed, d) washing the gel substance with water and subsequently impregnating it with a volatile, water-miscible solvent, e) freeing the anhydrous gel substance or gel substances obtained as in d) from the volatile, water-miscible solvent, f) burning out the organic constituents from the dry gel substance or the dry gel substances obtained as in c), g) sintering the product obtained as in f).
 4. The process according to claim 3 for producing a ceramic capillary frit, characterized by the following steps: a) mixing an aqueous solution of a ionotropically orientable polyanion, either with oxides, hydroxides or hydrated oxides, which are present in the form of a sol, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline oxides, hydroxides or hydrated oxides, which are present in suspension, of a metal selected from the group consisting of Al, Zr, Ti, and Nb, or with finely divided crystalline tricalcium phosphate or apatite which are present in suspension, b) coating the mixed sol obtained as in a) or the suspension obtained as in a) with a solution of a salt of a di- or trivalent cation in order to produce an ionotropic gel substance and cutting said gel substance into slices, c) compacting the gel substance by introducing it into electrolyte solutions which further enhance the syneresis of the polyelectrolyte which has originally formed, d) washing the gel substance with water and subsequently impregnating it with a volatile, water-miscible solvent, e) freeing the anhydrous gel slices obtained as in d) from the volatile, water-miscible solvent, f) burning out the organic constituents from the dry gel slices obtained as in e), g) sintering the product obtained as in f).
 5. The porous ceramic according to claim 1, wherein the porous ceramic is suitable for use as a catalyst or catalyst support.
 6. A composite material comprising a ceramic component comprising a granular ceramic material produced from the ceramic according to claim
 1. 7. The composite material of claim 6, wherein the composite material further comprises a synthetic resin and optionally a coupling agent.
 8. The composite material according to claim 7, wherein the composite material is suitable for use as a dental material.
 9. The porous ceramic according to claim 2, wherein the porous ceramic is suitable for use as a catalyst, a catalyst support, a reversible flow filter or as a monolithic sorbent suitable for use in a chromatography column.
 10. The composite material of claim 8, wherein the composite material is suitable for use as a dental cement.
 11. A porous ceramic material comprising at least one ceramic material prepared from a sol or finely divided crystals of an oxide, hydroxide or hydrated oxide of Al, Zr, Ti, or Nb or from a suspension of finely divided crystalline tricalcium phosphate or apatite, wherein the porous ceramic material has a porous structure comprising coparallel capillaries which are arranged in a hexagonal array.
 12. The porous ceramic material of claim 11, wherein the capillaries have a diameter of between about 1 and about 200 μm and adjacent capillaries are separated by a distance substantially equal to the capillary diameter. 